1. Field of the Invention
The present invention relates to a laser processing apparatus for performing laser processing to a workpiece such as a semiconductor wafer.
2. Description of the Related Art
In a semiconductor device fabrication process, a plurality of crossing division lines called streets are formed on the front side of a substantially disk-shaped semiconductor wafer to thereby partition a plurality of regions where a plurality of devices such as ICs and LSIs are respectively formed. The semiconductor wafer is cut along the streets to thereby divide the regions where the devices are formed from each other, thus obtaining a plurality of individual semiconductor chips. Further, an optical device wafer is provided by forming a plurality of optical devices including photodetectors such as photodiodes and light emitting devices such as laser diodes on the front side of a sapphire substrate. The optical device wafer is also cut along the streets to obtain the individual optical devices divided from each other, such as photodiodes and laser diodes, which are widely used in electric equipment.
As a method of dividing a wafer such as a semiconductor wafer and an optical device wafer along the streets, there has been proposed a method of applying a pulsed laser beam having an absorption wavelength to the wafer along the streets to thereby form laser processed grooves by ablation, thus dividing the wafer along the streets.
As another method of dividing such a wafer as mentioned above, there is a method including the steps of applying a pulsed laser beam having a transmission wavelength to the wafer in the condition where the focal point of the pulsed laser beam is set inside the wafer in an area to be divided, thereby continuously forming modified layers inside the wafer along the streets and next applying an external force along the streets where the modified layers are respectively formed to be reduced in strength, thereby dividing the wafer along the streets.
Such laser processing is performed by using a laser processing apparatus including workpiece holding means for holding a workpiece and laser beam applying means for applying a laser beam to the workpiece held by the workpiece holding means. The laser beam applying means is composed of laser beam oscillating means for oscillating a laser beam, focusing means for focusing the laser beam oscillated by the laser beam oscillating means onto the workpiece held by the workpiece holding means, and an optical system provided between the laser beam oscillating means and the focusing means and including power adjusting means for adjusting the power of the laser beam and a beam expander for adjusting the diameter of the laser beam (see Japanese Patent Laid-open No. 2006-108478, for example).